This invention relates to a vacuum capacitor to be used in an oscillator circuit of a supper power oscillator, a high frequency power circuit for a semiconductor producing apparatus, a tank circuit of an induction heating apparatus, or the like.
Vacuum capacitors are roughly structurally classified into a vacuum fixed capacitor whose electrostatic capacity value is fixed and a vacuum variable capacitor whose electrostatic capacity value is variable. An earlier technology vacuum variable capacitor is shown in FIG. 6 as a vertical sectional view. The vacuum variable capacitor includes an insulating cylinder 1 formed of ceramic or the like. The insulating cylinder 1 has one end to which a stationary-side flange 2 is installed, and the other end to which a movable-side flange 4 is installed through a sealing metal fitting 3, so that a vacuum container 5 is formed. The reference numerals 2a, 4a designate respectively female thread portions through which outside connection conductors are to be installed to the flanges 2, 4.
A plurality of cylindrical electrode plates which have respectively different diameters are coaxially disposed standing on the inner surface of the stationary electrode supporting plate 2 in such a manner as to be spaced a certain distance from each other, thereby forming a stationary electrode 6. Additionally, a center pin 8 formed of an insulating material is disposed standing at the central portion of the inner surface of the stationary-side flange 2 through a stationary guide 7. A cylindrical movable lead 9 is slidably fitted on the center pin 8. A movable electrode supporting plate 10 is installed to the movable lead 9. A plurality of cylindrical electrode plates having different diameters are coaxially disposed standing on the movable electrode supporting plates 10. Each cylindrical electrode plate is insertable into and withdrawable from a space between the adjacent cylindrical electrode plates of the stationary electrode 6 without contacting with the cylindrical electrodes of the stationary electrode, thereby forming a movable electrode 11. Additionally, a movable lead bolt section 9a is fixedly connected to the lower end of the movable lead 9 as a single member.
The movable-side flange 4 is formed at its central portion with a hole 4b defined by an inner peripheral portion of the flange 4. A cylindrical heat pipe 12 is disposed standing at the inner peripheral portion of the movable-side flange 4. A nut receiving section 13 is installed to the inner peripheral portion of the heat pipe 12. An adjusting nut 15 is rotatably installed through a bearing 14 to the nut receiving section 13. The adjusting nut 15 is formed at its inner peripheral portion with a female thread portion 15a which is engageable with the male thread portion 9b of the movable lead bolt 9a. The adjusting nut 15 is formed with a large-diameter hole section 15c which is contiguous with the female thread portion 15a through a step portion 15b. The movable lead bolt 9a is formed at its lower end portion with an axially extending female thread portion 9c with which an adjusting screw 16 is engaged, in which the a head portion 16a of the adjusting screw 16 is brought into engagement with the step portion 15b of the adjusting nut 15. The reference numeral 17 designates a bellows which is extensible conductor and define a vacuum side and an atmospheric side. The one end of the bellows 17 is attached to the movable electrode supporting plate 10 while the other end is attached to the movable-side flange 4.
With the above arrangement, in order to adjust the maximum electrostatic capacity value of the vacuum capacitor, first the adjusting nut 15 is slightly turned to the right to move the movable lead 9 downward from a position corresponding to the maximum electrostatic capacity value at which position the tip end of the movable lead bolt 9a is brought into contact with the tip end of the center pin 8, thereby adjusting the defined maximum electrostatic capacity value. Subsequently, the adjusting screw 16 is screwed into the female thread portion 9c until the head portion 16a of the adjusting screw 16 is brought into contact with the step portion 15b. Then, the adjusting screw 16 is fixed to the female thread portion 9c of the movable lead bolt 9a with an adhesive or the like. As a result, if the adjusting nut 15 is intended to be turned to the left from the position corresponding to the maximum electrostatic capacity value, it cannot be turned to the left since the head portion 16a of the adjusting screw is brought into contact with the step portion 15b. 
Adjustment of the electrostatic capacity of the vacuum capacitor is carried out as follows: The movable lead bolt 9a moves downward when the adjusting nut 15 is turned to the right, and moves upward when the adjusting nut 15 is turned to the left. This moves the movable electrode 11 upward and downward to vary the total facing area between the movable electrode 11 and the stationary electrode 6, thereby adjusting the electrostatic capacity.
The movable lead 9 receives a force for pushing it up under the pressure differential between a vacuum on the vacuum side and an atmospheric pressure on the atmospheric side. The adjusting nut 15 also receives the same force so as to generate a surface pressure at the nut receiving section 13, so that a large rotational torque is required to turn the adjusting nut 15. However, since the bearing 14 is provided between the nut receiving section 13 and the adjusting nut 15, turning of the adjusting nut is easy. Additionally, since the heat pipe 12 is provided, heat generated in the bellows 17 upon current-flowing is absorbed and radiated through the heat pipe 12, the movable-side flange 4 and the outside connection conductor, thereby prolonging the life of the bellows 17 and other members.
An earlier technology vacuum fixed capacitor is shown in FIG. 7 in the form of a vertical sectional view. The vacuum fixed capacitor includes an insulating cylinder 18 formed of ceramic or the like. Flanges 19, 20 formed of cupper are respectively provided at the opposite end sides of the insulating cylinder 18. The flange 19 is provided at its outer peripheral part with cylindrical peripheral section 19a installed to one end of the insulating cylinder 18, while the flange 20 is provided at its outer peripheral part with a cylindrical peripheral section 20a installed to the other end of the insulating cylinder 18. Thus, a vacuum container 24 is constituted by the insulating cylinder 18 and the flanges 19, 20.
Each of the flanges 19, 20 is provided with a projection 19a, 20b located at the central portion of the inner surface side thereof. A positioning pin fitting hole 19c, 20c is formed at the tip face of the projection 19a, 20b. A female thread portion 19d, 20d for installation of an outside connection conductor is formed at the central portion of the outer surface side of the flange 19, 20. A locating pin 21 formed of ceramic is provided between the projections 19b, 20b in such a manner that the opposite end sections of the pin 21 are fitted respectively in the fitting holes 19c, 20c of the projections 19b, 20b. 
A plurality of cylindrical electrode plates which have respectively different diameters are coaxially disposed standing on the inner surface of the flange 19 in such a manner as to be spaced a certain distance from each other, thereby forming a stationary electrode 22. Additionally, a plurality of cylindrical electrode plates having different diameters are coaxially disposed standing on the inner surface of the flange 20. Each cylindrical electrode plate is insertable into and withdrawable from a space between the adjacent cylindrical electrode plates of the stationary electrode 22 without contacting with the cylindrical electrode plates of the stationary electrode 22, thereby forming a stationary electrode 23. Thus, since the locating pin 21 is provided, the radial and axial distances between the stationary electrodes 22, 23 are made uniform.
In the vacuum capacitors shown in FIGS. 6 and 7, each of the stationary electrode 6, 22, 23 and the movable electrode 11 includes a plurality of thin and coaxial cylindrical electrode plates, in which the total of the electrostatic capacities each of which is produced by the facing cylindrical electrode plates corresponds to the electrostatic capacity of the whole vacuum capacitor. Here, the electrostatic capacity C per unit length L, of infinite coaxial cylindrical electrodes is represented by an equation (1) which can be applied to finite coaxial cylindrical electrodes.C=2πε0L/log (b/a)  (1)
where a is the radius of an inner cylindrical electrode plate, b is the radius of an outer cylindrical electrode plate, ε0 is the vacuum dielectric constant, and L is the length of the cylindrical electrode plates. In case that the electrode 6, 11, 22, 23 is constituted of a plurality of cylindrical electrode plates, the electrostatic capacity of the vacuum capacitor is calculated by totaling the values C each of which is obtained by the above equation (1), in which the voltage proof characteristics at this time is decided by the difference between the radius a of the inner cylindrical electrode plate and the radius b of the outer cylindrical electrode plate.
Prior art documents related to the invention of this application are Japanese Patent Provisional Publication Nos. 6-204082, 7-211588 and 8-97088.